Sweetpotato is the world's seventh most important food crop, and its storage roots provide high levels of digestible nutrients and it has a high plant biomass per hectare. In the early stages of the development of its roots, it forms colorless fibrous roots. As root development proceeds, some of these fibrous roots become pigmented and begin to swell, ultimately developing into storage roots.
Based on early anatomical studies on storage root morphogenesis (Kokubu, 1973, Bulletin of the Faculty of Agriculture, Kagoshima University 23, 1-126; Wilson and Lowe, 1973, Annuls in Botany 37, 633-643; Nakatani and Komeichi, 1991), a storage root has been defined as a root in which there is anomalous secondary cambial activity inside of a primary cambium. Both the linear growth and the promotion of the thickening growth of the storage root and/or the yield of storage roots have been shown to be affected by environmental factors, including soil temperature, humidity, light, photoperiod, carbon dioxide, and drought (Loretan et al., 1994, Advances in Space Research 14, 277-280; Hill et al., 1996, Acta Horticulturae 440, 25-30; Mortley et al., 1996, Acta Horticulturae 440, 31-36; Eguchi et al., 1998, Biotronics 27, 93-96; Pardales et al., 1999, Plant Production Science 2, 247-251; Kano and Ming, 2000, Environment Control in Biology 38, 113-120; van Heerden and Laurie, 2008, Physiologia Plantarum 134, 99-109).
As mentioned above, some of the fibrous roots ultimately develop into storage roots. Particularly, it is reported that the number of storage roots decreases at a soil temperature of 40° C. compared to at 25° C., whereas the number of storage roots increases at a soil temperature of 20-36° C. compared to at 13-31° C. (Pardales et al., 1999, Plant Prod. Sci. 2, 247-251).
To date, however, the molecular mechanism related to the development from fibrous roots to storage roots has not been reported. Regulating the development process from fibrous roots to storage roots will enable the production of sweetpotato to be increased.
Improvements in various molecular approaches have enabled the mining of genes involved in storage root development in the sweetpotato, resulting in the identification of a number of genes that are differentially expressed in developing storage roots (You et al., 2003, FEBS Letters 536, 101-105; Tanaka et al., 2005, Journal of Plant Physiology 162, 91-102). Based on the results of their comparison of the distribution of KNOX1 gene expression and endogenous trans-zeatin riboside (t-ZR) in sweetpotato roots, Tanaka et al. (2008) suggested three sweetpotato class 1 knotted1-like homeobox (KNOX1) genes as possible regulators of cytokinin levels in storage roots (Tanaka et al., 2008, Journal of Plant Physiology 165, 1726-1735).
Ku et al. (2008, Annals of Botany 102, 57-67) recently isolated IbMADS1 from sweetpotato and analyzed its functional role in storage root development using potato overexpressing IbMADS1. However, to date, due to the difficulty of generating transgenic sweetpotato plants, researchers have been unable to verify directly whether a sweetpotato gene is actually involved or not, in the formation or thickening growth of storage roots using sweetpotato gain and/or loss of function mutants.
Meanwhile, most high value-added storage root plants, such as ginseng, are cultivated for several years. During the growth period, storage roots are susceptible to infection by various pathogen fungi, resulting in rot in their roots before harvest time.
Particularly, it is now reported that the portion of uprooting due to root rotting is up to approximately 50% in the field for the six-year old ginseng that is representative of high value-added storage root plants in Korea. To minimize such damage to the cultivation of storage root plants, generation of a cultivar resistant to root rot is required, along with development of a cultivation method that shortens the cultivation period. Identification of the genes involved in the development of storage roots and functional characterization of the genes are prerequisites for the molecular breeding.
Therefore, in order to increase storage root production, there has been a need to develop transgenic root plants bearing high-numbered storage roots or early-maturing transgenic root plants using genes involved in the storage root development.